Many devices and systems include a number of different types of sensors that perform various monitoring and/or control functions. Advancements in micromachining and other microfabrication processes have led to the manufacturing of a wide variety of microelectromechanical systems (MEMS) sensors to perform these monitoring and/or control functions.
A MEMS sensor may be formed on a wafer that includes a substrate (also known as a handle layer), an insulating sacrificial layer of, for example, an oxide, overlying the substrate, and an active layer overlying the insulating layer. In general, a MEMS sensor typically includes a movable element, sometimes referred to as a proof mass, formed in the active layer. The movable element may be coupled to the substrate by a set of compliant members, also referred to as springs or flexures.
The compliant members typically suspend the movable element over the substrate in a neutral position. The neutral position may be parallel or not parallel to the coupled substrate. The compliant members function to keep the movable element in the neutral position until the selective application of force, due to some other means, causes a deflection thereof. Movement of the movable element deforms the compliant members, storing potential energy therein. The stored potential energy tends to return the movable element to its neutral position once the force is removed. By way of example, the movable element of a MEMS accelerometer moves when the MEMS accelerometer experiences acceleration. Motion of the movable element is converted via electronics into a signal having a parameter magnitude (e.g. voltage, current, frequency, etc.) that is dependent on the acceleration.
When designing MEMS sensors that call for low lateral stiffness (i.e., low stiffness in the intended direction of motion of the movable element), the compliant members typically employ a folded spring structure. FIG. 1 shows a top view of a prior art MEMS sensor 20. MEMS sensor 20 includes a movable element 22, distinguished in FIG. 1 by upwardly and rightwardly directed hatching. Movable element 22 is adapted for lateral motion in a direction 24. Anchors 26, distinguished in FIG. 1 by downwardly and rightwardly directed hatching, are coupled to an underlying substrate 28. MEMS sensor 20 further includes folded spring members 30 interconnected between anchors 26 and movable element 22. Anchors 26 and folded spring members 30 function to suspend movable element 22 above substrate 28. Lateral movement of movable element 22 in direction 24 may be detected by sense electrodes 32 proximate movable element 22, as known to those skilled in the art.
Unfortunately, folded spring members 30 are subject to twisting under high vertical loads such as mechanical shock in excess of, for example, 10 g, which can occur during testing and in use. Indeed, failures have occurred under such high vertical loads. For example, a vertical load (perpendicular to the plane view of MEMS sensor 20) applied at an attachment point 34 of folded spring member 30 with movable element 22 introduces a moment of force to folded spring member 30. The vertical load can result in twisting of folded spring members 30 at their attachment points 34 and/or at folds 36 of folded spring member 30. This twisting effectively reduces the vertical stiffness and can result in large vertical displacement of movable element 22. A problem with excessive vertical displacement is that movable element 22 can contact substrate 28 and stick to substrate 28, thus resulting in a MEMS sensor failure. This problem can be exacerbated in some folded spring designs due to asymmetries of the folded structures.